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JDK5新特性之二----新的格式化输出

摘要: JDK5新特性之二----新的格式化输出

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JDK5.0允许象C语言那样直接用printf()方法来格式化输出，并且提供了许多参数来格式化输入，调用也很简单：

System.out.format("Pi is approximately %f", Math.Pi);

System.out.printf("Pi is approximately %f", Math.Pi);

printf()和 format() 方法具有相同的功能. System.out 是 java.io.PrintStream的实例. PrintStream, java.io.PrintWriter, 和 java.lang.String 每个类都有四个新的格式化方法:

format( String format, Object... args);

printf( String format, Object... args);

format( Locale locale, String format, Object... args);

printf( Locale locale, String format, Object... args);

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同时，以前的formatter类也提供了更完善的方法来格式化，例如：

formatter.format("Pi is approximately %1$f," + "and e is about %2$f", Math.PI, Math.E);

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格式化元素的构成如下：

%[argument_index$][flags][width][.precision]conversion

其中：

argument_index是一个正整数，说明了参数的位置，1为取第一个参数

width表示输出的最小字母个数

precision代表数字的小数位数

conversion代表被格式化的参数的类型：

f float,

t time

d decimal

o octal

x hexadecimal

s general

c a Unicode character

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以下是个例子：

package format;

import java.util.Formatter;

public class UsingFormatter {

public static void main(String[] args) {

if (args.length != 1) {

System.err.println("usage: " +

"java format/UsingFormatter ");

System.exit(0);

}

String format = args[0];

StringBuilder stringBuilder = new StringBuilder();

Formatter formatter = new Formatter(stringBuilder);

formatter.format("Pi is approximately " + format +

", and e is about " + format, Math.PI, Math.E);

System.out.println(stringBuilder);

}

//控制台调用

java format/UsingFormatter %f

//输出

Pi is approximately 3.141593, and e is about 2.718282

//控制台调用

java format/UsingFormatter %.2f

//输出

Pi is approximately 3.14, and e is about 2.72

//控制台调用

java format/UsingFormatter %6.2f

//输出(有空格来填补长度)

Pi is approximately 3.14, and e is about 2.72

//控制台调用

java format/UsingFormatter %1$.2f

//输出

Pi is approximately 3.14, and e is about 3.14

//改变区域设置

package format;

import java.util.Formatter;

import java.util.Locale;

public class UsingFormatter {

public static void main(String[] args) {

if (args.length != 1) {

System.err.println("usage: " +

"java format/UsingFormatter <format string>");

System.exit(0);

}

String format = args[0];

StringBuilder stringBuilder = new StringBuilder();

Formatter formatter = new Formatter(stringBuilder,

Locale.FRANCE);

formatter.format("Pi is approximately " + format +

", and e is about " + format, Math.PI, Math.E);

System.out.println(stringBuilder);

}

//控制台调用

java format/UsingFormatter %.2f

//输出

Pi is approximately 3,14, and e is about 2,72

//采用format,printf的可替代写法

package format;

public class UsingSystemOut {

public static void main(String[] args) {

if (args.length != 1) {

System.err.println("usage: " +

"java format/UsingSystemOut <format string>");

System.exit(0);

}

String format = args[0];

System.out.format("Pi is approximately " + format +

", and e is approximately " + format, Math.PI, Math.E);

}

//控制台调用

java format/UsingSystemOut %.2f%n

//输出

Pi is approximately 3.14

, and e is about 2.72

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对时间的格式化用字母t来代表，通常在t后面加上特殊的字符来显示时间的某个部分：

tr hour and minute,

tA the day of the week

tB the name of the month

te the number of the day of the month

tY the year

//eg.

package format;

import java.util.Calendar;

public class FormattingDates {

public static void main(String[] args) {

System.out.printf("Right now it is %tr on " +

Calendar.getInstance());

}

//说明：“<”指示采用的参数为前一个被格式化的参数

//输出

Right now it is 01:55:19 PM on Wednesday, September 22, 2004.

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